Appendix IL 823
vertical spindle which revolves in a bracket set upon the slide-rest,
being there driven from a counter-shaft pulley overhead. The
whole apparatus is seen in Fig, 794, where A is the blank, B the
bracket, containing the cutter spindle c driven by the belt D, and
E a dividing plate which gives the correct pitch turn through the
worm wheel F.
Bevel-wheel Teeth.—It has already been explained at p. 753,.
that true bevel teeth can only be cut with a conical feed, and
that milling cutters are useless except for approximations, or
for roughing out before using an exact tool. (See Appendix V.+
Worm-wheel Teeth.—These also are cut-by many methods-
which more or less approach accuracy. The simplest way,,
though by no means a good one, is to rough out the spaces-
with a milling cutter, as on p. 58, and then to apply the worm,,
as there shewn, but using the file to trim down the thick portions-
of the teeth where marked by red ochre from the worm.
The second method is to use a hob (see p. 274). This tool is-
obtained by turning a steel worm of proper size and shape, cutting
out milling teeth upon it, backing these off at top and sides for
clearance angle, and finally hardening. Such a tool is highly
expensive to make, and there is considerable risk when hardening;,
and unless finished with an emery wheel is likely to be untrue.
Very few sizes can therefore be afforded, and wheels must be
kept to standard pitch. Again, supposing the hob provided,,
there are still two methods of applying it. The spaces are always-
roughed out with a cutter, in order to save the hob, and the
wheel being mounted freely on a stud, the hob is rotated while -